JP2005314789A - Rolling device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling device having the prolonged life by refining crystal grains in the material. <P>SOLUTION: The rolling device employs a high-carbon bearing steel containing 0.5 wt.% or more C in at least one of an inner member, an outer member and a rolling element as the base material, makes the surface layer of a rolling contact side of a finished product consist of former austenite crystal grains with an average size of 3 μm or smaller, and have a nitrogen concentration of 0.3 wt.% or higher by nitriding or soft nitriding the high-carbon bearing steel during heat treatment. The heat treatment comprises: the first step of heating the material to 800 to 1,000°C to austenitize the metallographic structure, and then converting it into a martensite structure by quenching it; the second step of nitriding or soft nitriding it at a temperature of an Ae1 transformation point or lower; the third step of heating it to an austenitic temperature range of 780 to 880°C, and quenching it; and the fourth step of tempering it. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention includes rolling bearings represented by rolling bearings such as ball bearings, angular contact ball bearings, cylindrical roller bearings, tapered roller bearings, and self-aligning roller bearings used in automobiles, general industrial machines, machine tools, steel machines, and the like. It relates to extending the life of the device.

  Usually, high-carbon chromium bearing steel is used as the material for rolling devices such as rolling bearings, but in recent years, there has been an increasing demand for longer life in order to cope with the harsh trend of usage conditions. Then, the lifetime is extended by increasing the cleanliness of the material. In addition, the bearing steel is usually subjected to quenching and tempering treatment, but may be subjected to carbonitriding and quenching treatment in order to extend the life.

Regarding the improvement of the cleanliness of the materials mentioned above, it is impossible to eliminate inclusions in the production of steel, and a special melting method is required to reduce the inclusions to the limit. Leads to. In addition, when the carbonitriding process is performed, a long-time carbonitriding process is required, which also increases the cost of the product.
Therefore, in order to solve such problems, it is conceivable to improve the rolling fatigue life by increasing the strength of the bearing steel. As a technique for improving the strength of the material, metallographically, (1) make crystal grains (old austenite grains after heat treatment completed) fine, (2) increase the amount of carbon and nitrogen solid solution, (3) Precipitation strengthening by carbide is mainly cited.

The method (2) corresponds to the carbonitriding process described above and has a cost problem. In the method (3), cementite is equivalent to a precipitate in ordinary bearing steel and is necessary for strengthening. Therefore, in order to improve the strength by precipitation hardening, it is necessary to add special elements such as Mo and V, which also increases the cost of the material.
The strengthening by the refinement of the crystal grain of the above (1) is known by the Hall Petch rule that the material strength increases in proportion to the -1/2 power of the crystal grain diameter d. Since the strength is improved simply by reducing the diameter, it has recently been attracting attention.

As an example of the refinement of crystal grains of bearing steel, for example, a process in which processing is performed in a hot temperature and a warm temperature region and quenching is performed to obtain crystal grains of 4 to 7 μm is disclosed. (For example, refer to Patent Document 1).
In addition, as an example in which the grain size of the bearing steel is refined without processing, for example, carbonitriding is performed, and further, the secondary quenching is performed at a low temperature of about 800 ° C. As for 5-6 micrometers, what was trying to extend the lifetime on the conditions in which foreign substances, such as clean oil bath lubrication and abrasion powder mix, is disclosed (for example, refer patent document 2).
Japanese Patent No. 2524156 JP 2003-226918 A

By the way, in recent years, the use conditions of the rolling device tend to be more severe, and it is desired to make the rolling device as long as possible by miniaturizing crystal grains such as bearing steel. In 1 and Patent Document 2, it is considered difficult to make crystal grains finer to 3 μm or less.
The present invention has been made in view of such a technical background, and an object of the present invention is to provide a rolling device that can further refine the crystal grains of the material to further extend the life.

In order to achieve the above object, the invention according to claim 1 is a rolling device in which a plurality of rolling elements are disposed between an inner member and an outer member so as to be able to roll in the circumferential direction.
At least one material of the inner member, the outer member, and the rolling element is a high carbon bearing steel having a C content of 0.5% by weight or more, and the average austenite surface layer of the finished product rolling surface The crystal grain size is 3 μm or less, the nitrogen (N) concentration of the surface layer is 0.3 wt% or more, and the ratio of the carbon (C) concentration to the nitrogen (N) concentration is C / N = 1.10 or less. It is characterized by.

  The invention according to claim 2 is the first step according to claim 1, in which the heat treatment is performed by heating the material to 800 to 1000 ° C. to austenite, and then rapidly cooling to a martensite structure, and the Ae1 transformation point or lower. A second step of nitriding or soft nitriding at a temperature of 5 ° C., a third step of heating to 780 to 880 ° C. in the austenite temperature range, quenching after plastic working, and a fourth step of tempering. It is characterized by that.

  The invention according to claim 3 is the first step according to claim 1, wherein the heat treatment is performed by heating the material to 800 to 1000 ° C. to austenite, and then rapidly marring to a martensite structure, and the Ae1 transformation point or less. A second step of nitriding or soft nitriding at a temperature of 5 ° C, a third step of heating and quenching at 780 to 880 ° C in the austenite temperature range, and a fourth step of tempering. To do.

  According to the present invention, at least one of the inner member, the outer member, and the rolling element is a high carbon bearing steel having a C content of 0.5% by weight or more, and the finished product rolling surface surface layer is formed. The prior austenite average crystal grain size is 3 μm or less, the nitrogen (N) concentration of the surface layer is 0.3 wt% or more, and the ratio of carbon (C) concentration to nitrogen (N) concentration is C / N = 1.10 or less. As a result, the grain size of the prior austenite after the completion of the heat treatment is refined to increase the material strength and strengthen the matrix structure, thereby enabling the rolling fatigue life to be extended.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram for explaining a heat treatment process of a rolling bearing according to the first embodiment of the present invention, FIG. 2 is a graph showing the relationship between the crystal grain size and the life ratio, and FIG. 4 is a photograph of the metal structure by a scanning electron microscope, FIG. 4 is a photograph of the metal structure by a scanning electron microscope of Comparative Example 5, FIG. 5 is a graph showing the relationship between nitrogen concentration and crystal grain size, and FIG. FIG. 7 is an explanatory diagram for explaining the heat treatment process of the rolling bearing according to the second embodiment of the present invention, and FIG. 8 is a graph showing the relationship between the crystal grain size and the life ratio. FIG. 9 is a graph showing the relationship between nitrogen concentration and crystal grain size, FIG. 10 is a graph showing the relationship between hardness and N / C, and FIG. 11 is the life ratio and N / C. It is a graph which shows the relationship.

In the first embodiment of the present invention, the crystal grain size of high carbon steel typified by high carbon chrome steel SUJ2 generally used for rolling bearings is set to be higher than that obtained when normal quenching and tempering treatments are performed. By reducing the size, rolling fatigue characteristics are improved.
The first feature of the grain refinement method according to the first embodiment of the present invention is to use nitrides and carbonitrides in order to increase the number of generation sites of austenite grains during austenitization. It is. Usually, in bearing steel, carbonitriding is performed as a method for increasing the nitrogen content, but carbonitriding is performed at 800 to 900 ° C., and NH ₃ that is a supply gas of nitrogen is decomposed. The nitrogen concentration cannot be increased effectively.

On the other hand, in this embodiment, attention is paid to nitriding or soft nitriding as a method for further increasing the nitrogen concentration. The nitriding or soft nitriding treatment is performed at 500 to 600 ° C., can effectively increase the nitrogen concentration, and nitrides and charcoal that serve as nucleation sites for recrystallization during austenitizing than when carbonitriding. The amount of nitride can be significantly increased.
A second feature is that after nitriding or soft nitriding, plastic working is performed in the austenite temperature range, and recrystallization of austenite is used to promote miniaturization.

  That is, in the rolling bearing according to the first embodiment of the present invention, at least one material of the inner ring (inner member), the outer ring (outer member), and the rolling element contains C content: 0.5% by weight. By using the above high carbon bearing steel and performing nitriding or soft nitriding treatment and plastic working at the time of heat treatment, the old austenite average crystal grain size of the finished rolling surface surface layer is 3 μm or less, and the nitrogen concentration of the surface layer is 0 By setting the content to 3% by weight or more, the grain size of the prior austenite after the heat treatment is refined to increase the material strength and strengthen the base structure, thereby making it possible to extend the rolling fatigue life.

  Then, as shown in FIG. 1, the heat treatment is performed by heating the material at 800 ° to 1000 ° C. for 20 to 60 minutes to form austenite, and then rapidly cooling to form a martensite structure, and the Ae1 transformation The second step of nitriding or soft nitriding for 3 to 48 hours, for example, at 500 to 600 ° C. below the point, and heating at 780 to 880 ° C. in the austenite temperature range for 10 to 30 minutes, processing rate 25 to 50 % Of the prior austenite average crystal grain size of the finished product rolling surface layer is 3 μm or less, and the nitrogen of the surface layer is provided with a third step of quenching after plastic working at 4% and a fourth step of tempering. The concentration can be 0.3% by weight or more.

Next, each step will be described in detail.
(First step)
A 1st process is a hardening process as a pre-processing, and is a process made into a martensitic structure. The quenching temperature is in the range of 800 to 1000 ° C. for 20 to 60 minutes, and it has a three-phase structure of martensite, retained austenite and cementite, or from 830 to 850 ° C. which is the standard quenching temperature of ordinary bearing steel Alternatively, the spheroidized cementite may be eliminated by quenching at a high 900 to 1000 ° C.

(Second step)
The second step is a nitriding or soft nitriding treatment step, which is performed at or below the Ae1 transformation point, for example, 500 to 600 ° C. for 3 to 48 hours. In this step, iron and nitrogen react to form iron nitride of Fe ₄ N, Fe ₃ N, and Fe ₂ N depending on the nitrogen concentration. Nitriding or soft nitriding can enrich nitrogen at a concentration as high as about 15% by weight that cannot be obtained by carbonitriding, and a very large amount of very fine nitride or carbonitride is formed. . The fine nitrides or carbonitrides serve as nucleation sites for austenite grains in the next third step and promote miniaturization.

By setting the nitrogen concentration at the completion of nitriding or soft nitriding to 0.5% by weight or more, fine nitride or carbonitride effective for refining crystal grains is formed.
As the bearing steel, a spheroidized annealed material is usually used as a standard, but when the spheroidized annealed material is subjected to nitriding or soft nitriding treatment, refinement of crystal grains does not proceed. This is because the spheroidized annealed material has two phases of ferrite and spheroidized cementite, but when spheroidized annealed material of low alloy steel such as bearing steel represented by SUJ2 is subjected to nitriding or soft nitriding treatment This is because the nitride formed is called a so-called white layer and is formed in a layer on the surface, so that fine nitride and carbonitride effective for refining crystal grains are not formed. Therefore, in this embodiment, as described in the first step, it is essential that the state of the structure as the previous stage of performing nitriding or soft nitriding is martensite as a basic constituent phase.

(Third step)
The third step is an austenitization and plastic working step in the austenite region, which corresponds to a substantial refinement step of crystal grains, and is 780 to 880 ° C., more preferably 780, higher than the A1 transformation point. Performed at ˜840 ° C. Examples of the plastic working method in the austenite temperature range include rolling forming by a rolling mill and hot forging.
Since the austenite grains are formed with the fine nitrides precipitated in the second step as nuclei, the crystal grains are remarkably made finer than when the spheroidized annealing material is austenitized. Even if austenite is used alone, crystal grain refinement progresses, but plastic working is performed as a step for further promoting refinement.

  By this plastic working, austenite grains are deformed to introduce work strain, and recrystallization occurs due to this strain energy. By setting the end temperature of the plastic working to the recrystallization temperature of austenite, recrystallization proceeds immediately after the introduction of working strain, new austenite grains are formed, and refinement of crystal grains proceeds. Since recrystallization of austenite is completed in a short time, a martensitic structure with fine prior austenite grains can be obtained by quenching immediately after plastic working.

Since the austenitizing temperature is higher than the nitriding or soft nitriding temperature, the diffusion of the surface nitrogen into the interior proceeds in the heating and holding stage of the third step, and the surface nitrogen concentration becomes after the completion of the nitriding or soft nitriding treatment. It will be lower. Although details will be described later, if a nitrogen concentration of 0.3% by weight or more can be maintained at the completion of plastic working in the austenite temperature range, nitride and carbonitride that suppress the growth of austenite crystal grains remain during austenite heating. Therefore, the crystal grain size can be kept fine.
After quenching in the third step, the fourth step is tempered (160 to 240 ° C., more preferably 160 to 180 ° C.), subjected to grinding, and used as a bearing.

By passing through the above first to fourth steps, a normal quenching and tempering treatment using a spheroidized annealing material as a starting material is a martensitic structure having a crystal grain size of about 15 to 20 μm. In the form, the crystal grain size can be reduced to 3 μm or less, and the material strength is improved and the life is extended by the refinement of the crystal grains.
In addition, since the surface nitrogen concentration at the time of completion of heat treatment is 0.3% by weight or more, the base structure is strengthened, and the effect of extending the life is greater than in the case of crystal grain refinement alone. Longer service life is possible even underneath.

Next, with reference to FIG. 7, the heat processing process of the rolling bearing which is the 2nd Embodiment of this invention is demonstrated.
In the second embodiment of the present invention, as in the first embodiment, the crystal grains of the high carbon steel represented by the high carbon chromium steel SUJ2 that is generally used for rolling bearings are used. Rolling fatigue characteristics are improved by making it finer than when quenching and tempering.
The first feature of the crystal grain refining method in this embodiment is that, as in the first embodiment, nitrides and carbonitrides are used to increase the number of austenite grain nuclei during austenitization. Is used. Normally, in bearing steel, carbonitriding is performed as a method for increasing the nitrogen content, but carbonitriding is performed at 800 to 900 ° C., and NH ₃ that is a supply gas of nitrogen is decomposed. The nitrogen concentration cannot be increased effectively.

  On the other hand, in this embodiment, attention is paid to nitriding or soft nitriding as a method for further increasing the nitrogen concentration. The nitriding or soft nitriding treatment is performed at a temperature of, for example, 500 to 600 ° C. below the temperature of the Ae1 transformation point, and the nitrogen concentration can be increased to several weight%. The amount of nitrides and carbonitrides that become nucleation sites can be remarkably increased. Since these nitrides and carbonitrides suppress the growth of crystal grains, it is considered that fine crystal grains are generated.

Further, as a second feature, after nitriding or soft nitriding after quenching, a tempering process is performed again through a quenching process.
That is, in the rolling bearing according to the second embodiment of the present invention, at least one material of the inner ring (inner member), the outer ring (outer member), and the rolling element contains C content: 0.5% by weight. By using the above high carbon bearing steel and performing nitriding or soft nitriding treatment and re-quenching treatment after quenching during the heat treatment, the old austenite average crystal grain size of the finished rolling surface surface layer is 3 μm or less, By making the nitrogen concentration 0.3% by weight or more, the grain size of the prior austenite after heat treatment is refined to increase the material strength and strengthen the base structure, thereby enabling the rolling fatigue life to be extended. .

  Then, as shown in FIG. 7, the heat treatment is performed by heating the material at 800 ° to 1000 ° C. for 20 to 60 minutes to form austenite, and then rapidly cooling to form a martensite structure, and the Ae1 transformation A second step of performing nitriding or soft nitriding for 3 to 48 hours, for example, at 500 to 600 ° C. below the point temperature, and 10 to 10 at 780 to 840 ° C. in the austenite temperature range, more preferably 780 to 840 ° C. By providing the third step of heating and quenching for 30 minutes and the fourth step of tempering, the prior austenite average crystal grain size of the finished product rolling surface surface layer is 3 μm or less without using a processing device. The nitrogen concentration of the surface layer can be 0.3% by weight or more.

Next, each step will be described in detail.
(First step)
A 1st process is a hardening process as a pre-processing, and is a process made into a martensitic structure. The quenching temperature is in the range of 800 to 1000 ° C. for 20 to 60 minutes, and has a three-phase structure of martensite, retained austenite and cementite, or 830 to 850 ° C., which is a standard quenching temperature of ordinary bearing steel. The spheroidized cementite may be eliminated by quenching at a higher temperature of 900 to 950 ° C.

(Second step)
The second step is a so-called nitriding or soft nitriding treatment step, which is performed at the Ae1 transformation point or lower, for example, at 500 to 600 ° C. for 3 to 48 hours. In this step, iron and nitrogen react, and iron nitrides of Fe ₄ N, Fe ₃ N, and Fe ₂ N are formed on the surface according to the nitrogen concentration. Nitriding or soft nitriding can enrich nitrogen at a high concentration of up to about 15% by weight that cannot be obtained by carbonitriding, and a very large amount of very fine nitride or carbonitride is formed. . This fine nitride or carbonitride serves as a nucleation site for austenite grains in the next third step and promotes miniaturization.

The nitrogen concentration at the time of completion of nitriding or soft nitriding is at least 0.3 wt% or more (preferably 0.5 wt% or more), so that fine nitride or carbonitride effective for refining crystal grains Is formed.
As the bearing steel, a spheroidized annealing material is normally used as a standard. However, when the spheroidizing annealed material is subjected to nitriding or soft nitriding treatment, miniaturization is difficult to proceed as compared with the quenched material. This is considered to be a result of the carbon generation state being largely involved in the diffusion of nitrogen and the generation of nitrides and carbonitrides.

(Third step)
The third step is a quenching treatment from the austenite region, which substantially corresponds to a crystal grain refining step, and is 780 to 880 ° C, more preferably 780 to 840 ° C, higher than the A1 transformation point. Done.
Since austenite grains are formed with the fine nitrides precipitated in the second step as nuclei, the refining progresses remarkably compared to the case where the spheroidized annealing material is austenitized.
Since the austenitizing temperature is higher than the nitriding or soft nitriding temperature, in the heating and holding stage in the third step, diffusion of surface nitrogen into the interior proceeds, so the surface nitrogen concentration is nitriding or soft nitriding. Lower than after completion. Although details will be described later, if a nitrogen concentration of 0.3% by weight or more can be maintained in the austenite temperature range, nitride and carbonitride that suppress the growth of austenite crystal grains can be left at the time of austenite heating. It is possible to keep the crystal grain size fine.

After the third step, a tempering process (160 to 240 ° C., more preferably 160 to 180 ° C.), which is the fourth step, is performed, subjected to grinding, and used as a bearing.
In a normal quenching process using a spheroidized annealing material as a starting material, the crystal grain size is a martensite structure of about 15 to 20 μm. By passing through the first to fourth steps described above, the processing apparatus is Without using it, it becomes possible to refine the crystal grains of the surface layer of the finished product rolling surface to 3 μm or less only by heat treatment, and the refinement of the crystal grains improves the material strength and extends the life.

In addition, since the nitrogen concentration in the surface layer is 0.3% by weight or more, the base structure is strengthened, the effect of extending the life is greater than in the case of only miniaturization, and the life is extended even under the contamination with foreign matter. Is possible.
FIG. 10 shows the relationship between the nitrogen (N) concentration (wt%) and the carbon (C) concentration (wt%) of the surface layer after the heat treatment in each of the above embodiments = N / C and hardness. .
As is apparent from the figure, when N / C exceeds 1.10, the carbon concentration decreases in the diffusion region of nitrogen, so that the hardness decreases, and as a result, the rolling life decreases. It can be seen that when N / C is in the range of 1.10 or less, more preferably in the range of N / C in the range of 0.3 to 1.10, the solid solution strengthening is significant and the rolling life is improved.

In addition, the bearing type in each of the above embodiments is not limited, and is applicable to rolling bearings such as ball bearings, angular ball bearings, cylindrical roller bearings, tapered roller bearings, and self-aligning roller bearings and other rolling devices. Can be applied, and the same effects can be obtained in any case.
In each of the above embodiments, for example, the surface layer is 0.1 mm from the surface, but for example, the depth from the surface to 0.5 mm is the surface layer, and the crystal grain size, nitrogen concentration, carbon concentration, and nitrogen concentration It is more preferable that the ratio to is in the above range.

(First Example: Corresponding to the first embodiment)
In order to confirm the effect of the present invention, the following experiment was conducted.
A test piece was prepared using high-carbon chromium steel SUJ2. The starting material was a spheroidized annealing material, and the surface nitrogen concentration after nitriding was in the range of 0.5 to 12% by weight. Table 1 shows the heat treatment process of each specimen. After the heat treatment, grinding was performed.

These test pieces were subjected to a thrust life test. For the life test, Mori-type Thrust Roller used a life tester. The test conditions are as follows.
(Life test conditions)
Load: 4900 MPa
Rotational speed: 300min ^-1
Lubricating oil: # 68 Turbine oil Foreign matter Composition: Fe ₃ C powder Hardness: HRC52
Particle size: 74-147 μm
Inclusion amount: 300ppm in lubricating oil
Each test piece is tested for n = 10, and the number of repeated stress cycles (life) until flaking occurs is investigated to create a Weibull plot. From the results of each Weibull distribution, each L ₁₀ Lifetime was sought.

  The crystal grain size (former austenite grain size) of the finished product rolling surface of each test piece is etched with a mixed solution of ferric chloride and hydrochloric acid to reveal only the prior austenite grain boundary, and a scanning electron microscope The crystal grain size was measured by the intercept method using the photographs taken at a magnification of 2000 to 10,000 times and averaged. The nitrogen concentration in the surface layer was measured using an electron beam microanalyzer (EPMA).

The test results are also shown in Table 1. The life ratio in Table 1 is indicated by the ratio of L ₁₀ life of normal quenching and tempering in Comparative Example 5 as 1.
As can be seen from Table 1, in Examples 1 to 7 of the present invention, the nitrogen concentration of the surface layer is 0.3% by weight or more and the crystal grain size ( The prior austenite grain size) is refined to 3 μm or less, and the effect of extending the life under the contamination with foreign matters is clear. FIG. 2 shows the relationship between the crystal grain size and the life ratio. FIG. 2 shows that the life ratio is improved when the crystal grain size is 3 μm or less. As an example, FIG. 3 shows a metal structure by a scanning electron microscope of Example 2 of the present invention, and FIG. 4 shows a metal structure by a scanning electron microscope of Comparative Example 5.

  FIG. 5 shows the relationship between the nitrogen concentration of the surface layer and the crystal grain size in Examples 1 to 7 and Comparative Example 3 of the present invention. From the figure, the nitrogen concentration of the surface layer is 0.3 weight. When it is less than%, it can be seen that the crystal grain size is larger than the upper limit of 3 μm of the present invention. This is because when the nitrogen concentration of the surface layer is less than 0.3% by weight, the abundance ratio of nitride and carbonitride that suppress the growth of crystal grains during austenitization is reduced, and the pinning effect is reduced. .

From the above results, in the present invention, the nitrogen concentration of the surface layer is 0.3% by weight or more, and the average crystal grain size of prior austenite is 3 μm or less.
Comparative Example 1 is a case where the quenching step of the first step is not performed, and the refinement of crystal grains is not sufficiently advanced, and the life extension effect is small. This is because, as described above, if the martensite structure is not used before the nitriding treatment, fine nitrides and carbonitrides that become nucleation sites of austenite crystal grains cannot be obtained.

Comparative Example 2 is a case where the plastic working rate in the austenite temperature range in the third step is less than 25%, and the processing strain necessary for the refinement of crystal grains cannot be obtained, so the crystal grain size exceeds 3 μm. ing.
FIG. 6 shows the relationship between the plastic working rate and the crystal grain size in the austenite temperature range. From the figure, in order to reduce the crystal grain size to 3 μm or less, which has the effect of extending the life, plastic working is performed at a working rate of 25% or more. It turns out that it needs to be added. Moreover, when obtaining a fine crystal grain stably, it is set as 30% or more of workability.

  As the processing rate of plastic processing increases, the refinement of crystal grains progresses proportionally, but even if processing is performed at a processing degree exceeding 50% in thickness ratio, the degree of miniaturization decreases. Moreover, in order to add large processing, the apparatus becomes large and the cost increases. Therefore, in the present invention, the rate of plastic working in the austenite temperature range is 25 to 50%, and the range that can be stably manufactured at low cost is 30 to 50%.

Comparative Example 4 is a case where the first step and the second step are not performed, and the material is only subjected to plastic working after austenizing, but the effect of fine nitride and carbonitride is obtained. Therefore, the crystal grains are not sufficiently refined and the life extension effect is small.
Comparative Example 6 is a case where the conventional spheroidized annealing material is carbonitrided and then subjected to quenching treatment and secondary quenching at 790 to 820 ° C., but the crystal grain size is 5 μm or more. It is larger than the range, and the life extension effect is smaller than the embodiment of the present invention.

(Second example: corresponding to the second embodiment)
In order to confirm the effect of the present invention, the following experiment was conducted.
A test piece was prepared using high-carbon chromium steel SUJ2. The starting material is a spheroidized annealing material. The tempering step was performed at 160 to 240 ° C. for 2 hours in any case. The surface nitrogen concentration after nitriding was in the range of 0.5 to 12% by weight. Table 2 shows the heat treatment process of each test piece. After the heat treatment, grinding was performed.

A thrust life test was conducted using these test pieces. A forest thrust rolling life tester was used for the life test. The test conditions are as follows.
(Life test conditions)
Load: 4900 MPa
Rotational speed: 300min ^-1
Lubricating oil: # 68 Turbine oil Foreign matter Composition: Fe ₃ C powder Hardness: HRC52
Particle size: 74-147 μm
Inclusion amount: 300ppm in lubricating oil
Each test piece is tested for n = 10, and the number of repeated stress cycles (life) until flaking occurs is investigated to create a Weibull plot. From the results of each Weibull distribution, each L ₁₀ Lifetime was sought.

  The average crystal grain size of the finished rolling surface surface layer of each test piece was etched with a solution based on ferric chloride to reveal only the prior austenite grain boundaries, and 2000 with a scanning electron microscope. The crystal grain size was measured using a photograph taken at a magnification of ˜3000. The crystal grain size was measured by the intercept method and averaged. The nitrogen concentration in the surface layer was measured using an electron beam microanalyzer (EPMA).

The test results are also shown in Table 2. The life ratio in Table 2 is shown by the ratio of L ₁₀ life of normal quenching and tempering of Comparative Example 5A as 1.
As can be seen from Table 2, in Examples 1A to 7A of the present invention, when the heat treatment step according to the present invention is performed, the nitrogen concentration of the surface layer is 0.3 wt% or more, and the crystal grain size Is refined to 3 μm or less, and the effect of extending the life under lubrication mixed with foreign matters is clear. FIG. 8 shows the relationship between the crystal grain size and the life ratio.

  FIG. 9 shows the relationship between the nitrogen concentration of the surface layer and the crystal grain size in Examples 1A to 7A and Comparative Examples 2A to 4A of the present invention. When it is less than 3% by weight, it can be seen that the crystal grain size is larger than 3 μm which is the upper limit of the present invention. This is because when the nitrogen concentration is less than 0.3% by weight, the abundance ratio of nitrides and carbonitrides that suppress the growth of crystal grains at the time of austenitization is reduced, and the pinning effect is reduced.

From the above results, in the present invention, the nitrogen concentration of the surface layer is 0.3% by weight or more, and the average crystal grain size of prior austenite is 3 μm or less.
Comparative Example 1A is a case where the quenching process of the first step is not performed, and the miniaturization is not sufficiently advanced, and the effect of extending the life is small. As described above, if the martensite structure is not used before nitriding, fine nitrides and carbonitrides that become nucleation sites of austenite crystal grains cannot be obtained, and the pinning effect during quenching is obtained. This is probably because it is small.

(Third example: corresponding to the first and second embodiments)
SUJ2 was used, and the starting material was a spheroidized annealing material. The tempering step was performed at 160 to 240 ° C. for 2 hours in any case. The surface nitrogen concentration after nitriding was 0.5% by weight or more. Table 3 shows the details of the heat treatment process for each test piece. Note that the ratio of nitrogen (N) concentration (% by weight) to carbon (C) concentration (% by weight) in the surface layer of the finished product rolling surface is controlled by changing the nitriding treatment time and grinding allowance. went.

A thrust type life test was conducted using this test piece. The forest thrust rolling life tester was used for the test. The test conditions are as follows.
Load: 8900N
Rotational speed: 1000min ^-1
Lubricating oil: # 68 Turbine oil The life of each sample is tested by n = 10, and the number of repeated repeated stresses (life) until the point at which peeling occurs is investigated to create a Weibull plot. From each, the L ₁₀ life was determined.

The average grain size of the finished rolling surface surface layer of each test piece was etched with a solution based on ferric chloride to reveal only the prior austenite grain boundaries, and 2000 to 3000 times with a scanning electron microscope. Using the photograph taken in step 1, the crystal grain size was measured by the intercept method and averaged. The nitrogen concentration was measured using an electron beam microanalyzer (EPMA).
The test results are also shown in Table 3. The life test results in the table are shown in a ratio where the L ₁₀ life of normal quenching and tempering in Comparative Example 6B is 1.

  As is apparent from Table 3, Examples 1B to 7B, which are examples of the present invention, have the prior austenite average crystal grain size of the surface layer of the finished product rolling surface of 3 μm or less by performing the heat treatment step according to the present invention described above. It can be seen that the effect of extending the life is large when the nitrogen concentration is 0.3 wt% or more and the value of N / C is 1.10 or less, more preferably 0.30 to 1.10. FIG. 11 shows the relationship between N / C and lifetime. As can be seen from the figure, it can be seen that the effect of extending the life is obtained when the value of N / C is 1.10 or less, more preferably 0.30 to 1.10.

In addition, in each said Example, although the example of the high carbon steel chromium bearing steel SUJ2 was shown, steel types are not limited to this, In addition to SUJ3, it is steel of 0.5 weight% or more in carbon amount, and a structure If is a steel type composed of martensite, cementite, and retained austenite, the same effect can be obtained.
In each of the above-described embodiments, only the gas nitriding is shown as the nitriding or soft nitriding treatment method, but it is not particularly limited, and the same method can be used when using tuftride treatment, ion nitriding, gas soft nitriding or the like. An effect is obtained.

It is explanatory drawing for demonstrating the heat processing process of the rolling bearing which is the 1st Embodiment of this invention. It is a graph which shows the relationship between a crystal grain diameter and a lifetime ratio. 3 is a photograph of a metal structure by a scanning electron microscope of Example 2. 7 is a photograph of a metal structure by a scanning electron microscope of Comparative Example 5. It is a graph which shows the relationship between nitrogen concentration and a crystal grain diameter. It is a graph which shows the relationship between a plastic working rate and a crystal grain diameter. It is explanatory drawing for demonstrating the heat processing process of the rolling bearing which is the 2nd Embodiment of this invention. It is a graph which shows the relationship between a crystal grain diameter and a lifetime ratio. It is a graph which shows the relationship between nitrogen concentration and a crystal grain diameter. It is a graph which shows the relationship between hardness and N / C. It is a graph which shows the relationship between a life ratio and N / C.

Claims (3)

In the rolling device in which a plurality of rolling elements are arranged so as to roll in the circumferential direction between the inner member and the outer member,
At least one material of the inner member, the outer member, and the rolling element is a high carbon bearing steel having a C content of 0.5% by weight or more, and the average austenite surface layer of the finished product rolling surface The crystal grain size is 3 μm or less, the nitrogen (N) concentration of the surface layer is 0.3 wt% or more, and the ratio of the carbon (C) concentration to the nitrogen (N) concentration is C / N = 1.10 or less. A rolling device characterized by.   In the heat treatment, after the material is heated to 800 to 1000 ° C. and austenitized, then a first step of forming a martensite structure by rapid cooling and a second step of nitriding or soft nitriding at a temperature equal to or lower than the Ae1 transformation point And a third step of heating to 780 to 880 ° C. in the austenite temperature range and quenching after plastic working, and a fourth step of tempering. apparatus.   In the heat treatment, the raw material is heated to 800 to 1000 ° C. to be austenite, and then a first step of forming a martensite structure by rapid cooling, and a second step of nitriding or soft nitriding at a temperature equal to or lower than the Ae1 transformation point. The rolling device according to claim 1, further comprising: a third step of heating to 780 to 880 ° C. in the austenite temperature range and quenching, and a fourth step of tempering.